Todo list for DGMulti solvers

This is intended to track additional tasks for solvers on simplicial (and more general) meshes

- [x] consider the use of `matmul!` from Octavian.jl - are we OK with adding this as an additional dependency?
- [x] Add Plots to `test/Project.toml` and enable all tests once upstream issues are fixed
- [x] switch to dispatch instead of using `UniformScaling` interpolation/projection matrices for nodal SBP methods https://github.com/trixi-framework/Trixi.jl/pull/647#discussion_r661498313
- [x] Add solver for tetrahedral meshes (now supported in StartUpDG!)
- [x] add flux differencing for modal and SBP triangular DG
- [x] add flux differencing for modal tetrahedral DG
- [x] add sparsity-optimized flux differencing for tensor product DG (see https://github.com/trixi-framework/Trixi.jl/pull/757)
- [x] add visualization for `DGMulti` (see https://github.com/trixi-framework/Trixi.jl/pull/755)

Solvers:
- [x] specialize volume integral on non-affine/curved meshes (do not use split forms by default)
- [x] add GSBP solver type for Gauss nodes (#781, https://github.com/trixi-framework/Trixi.jl/pull/927)
- [x] optimize GSBP solver using sparse `mul!` (see https://github.com/trixi-framework/Trixi.jl/pull/927#issuecomment-954308546) or tensor product operations for face interpolation
- [x] decide whether we want to implement generalized SBP solvers on simplices (no for now, it is probably not much more efficient than `Polynomial()` solvers)
- [x] use WADG (generalized mass lumping) for `invert_jacobian` on appropriate meshes.
- [ ] Add 3D tetrahedral SBP operators (needs upstream changes in StartUpDG)

Analysis
- [x] normalize L2 errors in analysis (https://github.com/trixi-framework/Trixi.jl/issues/862)

Meshes:
- [ ] convert between `UnstructuredQuadMesh` and `AbstractMeshData` types for use with quad/hex solvers.
- [ ] Convert between `CurvedMesh` and `AbstractMeshData` for uniform meshes
- [x] #961
- [x] Mark`VertexMappedMesh` for deprecation, replace with something more descriptive (e.g., `MeshDataMesh` or `DGMultiMesh`)

Nonconservative terms:
- [x] Added for `approximation_type = Polynomial()`
- [x] Added for `approximation_type = SBP()`
- [x] Add new capabilities to the overview in the docs

Feature requests/suggestions welcome!